In current connectorized electrostatic discharge (ESD) protection modules or accessories, typical ESD protection devices—such as Schottky diodes—are generally positioned such that they shunt to ground. A common approach involves placing the ESD device on a substrate having a microstrip or co-planar transmission line structure with either end-launch or flange-mount connectors. Among the drawbacks posed by such an approach, however, is the inherent loss due to the change of propagation mode, i.e. from coaxial to either microstrip or co-planar and then back to coaxial. For implementations that require better than ¾ dB of insertion loss and 15 dB of input return loss up to 40 GHz, for example, the loss due to the propagation mode conversion is too high and makes such approaches impractical at best.
A radial attachment method has been attempted for beam lead devices in which a small portion of a coaxial cable is removed to create a flat surface from the center conductor to the outer edge of the shield conductor. Two different means of attaching the beam lead diodes have been employed: conductive epoxy attachment and thermosonic bonding. Among the issues presented by such methods, however, is center pin conductor rotation due to over-torqueing, which would usually cause the beam lead device to break.
Also, subsequent experiments have shown that the electrical properties of the conductive epoxy material are changed after discharging large ESD current through them, thus posing a reliability issue. As for thermosonic bonding, bondable gold surfaces are required. Repeatable and reliable selective gold platting of the surfaces of the exposed plane of the coaxial cable where the bead device(s) needed to be bonded have not yet been achieved at worst and have low yield at best.
Connecting two coaxial transmission lines by means of planar blind mating where the conductive cross sections are pressed and held against each other requires a certain amount of elasticity at the junction in order to allow the expansion and contraction of the coaxial conductors, e.g., due to temperature variation, while maintaining a good connection. There are various ways of adding the compliance to the interface of the coaxial line. These can include integral spring elements such as spring-loaded connectors or adding spring elements to the mating surfaces.
Accordingly, there remains a need for an improved ESD limiting device, particularly with regard to coaxial line transmissions.